1. Field of the Invention
The invention relates to a paper making machine, and to a method for the manufacture of a material web such as in particular a paper or cardboard web, in which the material web to be manufactured is transferred on the surface of a transfer means to a take-off position, where a preferably air-permeable belt having an at least substantially open surface is guided over a take-off roll or the like and takes off the material web from the surface of the transfer means.
2. Discussion of the Background Information
The disturbing influences of the air movements on the running of the paper web increase with increasing speeds of the paper making machines. In this respect the edges of the web are in particular increasingly loaded, which leads to web edge fluttering which can lead to faults in the paper and indeed to tears. The runability and the efficiency of the paper making machine are thus greatly impaired. As a consequence it is necessary for the paper web to be fully supported during its run through the respective paper making machine, in particular on passing through the wet section, and free drafts should be avoided.
Various measures are already known for this purpose. Thus, in a paper making machine known from U.S. Pat. No. 4,359,828 an endless transfer belt is provided which runs through pressing gaps provided at a central roll of a compact pressing section. The paper web is located here at the outer side of the transfer belt. It is fully supported up to a transfer point and transferred without a free draft to a drying screen. The transfer belt in this arrangement has a substantially lower porosity than, for example, a pressing felt that is provided, or the dryer screen.
A transfer belt of this kind having a lower porosity in comparison with the porosity of the felts and screens is also used in a paper making machine known from U.S. Pat. No. 4,483,745. The porosity of this transfer belt is intended to be so low that it is practically water-impermeable. Belts of this kind have the advantage that with them, in comparison to press felts, the return wetting of the web is reduced or indeed avoided.
Such water-impermeable or substantially water-impermeable belts however give rise to further problems. Thus, on the one hand, the take-off of the paper web from a belt having a relatively closed surface and the transfer to the open dryer screen is problematic, and it is also difficult to keep the belt clean.
In a paper making machine which is known from example from U.S. Pat. No. 5,240,563, or from U.S. Pat. No. 5,534,116, the web transfer is to be assisted in that the transfer belt is deflected with the web, for example by deflection means, ahead of a suction roll around which the drying screen is guided. Alternatively a cylinder can be provided in the transfer belt loop with a suction device being provided in the region of the cylinder and downstream thereof which leads the web away from the transfer belt to the drying screen. These known proposals bring, amongst other things, the disadvantage that the contact between the web and the drying screen is produced solely by the dipping of the cylinder into the transfer belt, or by the dipping of the suction roll into the paper carrying transfer belt. As a consequence an enhancement of the contact can only be achieved by a further dipping in of the relevant elements.
This in turn brings the disadvantage with it that the tension of the transfer belt is impaired. Moreover, the relevant element undergoes non-uniform bending deflection over the width of the machine, which leads to different pressures between the web and the dying screen. Thus a non-uniform transfer takes place when considered over the machine width or the pressure cannot be increased to the degree required for reliable transfer.
Moreover, the means used in the previously known paper making machines for the cleaning of the transfer belt are inadequate. These also bring about a relatively pronounced mechanical loading of the belt, which is in particular to be attributed to the bending change which takes place in the so-called conditioning nip. Finally, the use of contacting cleaning elements such as scrapers and brushes leads to a wearing of the surface of the belt. This is in particular disadvantageous when the surface has a functional roughness, such as is for example the case with a paper making machine known from EP-B1-0 576 115.